Stop passing -Zincremental-verify-ich

Author: xmakro

collector/src/bin/rustc-fake.rs

@@ -75,14 +75,19 @@ fn main() {
     args.push(OsString::from("-Adeprecated"));
     args.push(OsString::from("-Aunknown-lints"));
 
-    // This forces incremental query hash verification on. Currently, rustc
-    // hashes 1/32 of queries loaded from disk without this flag, but that 1/32
-    // is based on the (expected) hash of the data, which can vary from build to
-    // build, adding a source of noise to our measurements, which we prefer to
-    // avoid. rustc-perf can accept the higher cost of always verifying hashes,
-    // and we currently prefer to avoid exposing a means of hard-disabling
-    // verification.
-    args.push(OsString::from("-Zincremental-verify-ich"));
+    // We deliberately do not pass -Zincremental-verify-ich. rustc-perf used to
+    // force it on every invocation because rustc selected the ~1/32 of
+    // incremental cache values it verifies by their (expected) value hash,
+    // which varies from build to build and moved the verification cost around
+    // between the two artifacts being compared. Since rust-lang/rust#160130
+    // the subset is selected by key fingerprint and session count, both of
+    // which are stable here: `determinism_env` pins the rustc version and both
+    // artifacts run the same scenario sequence from a fresh incremental
+    // directory, so both sides of a comparison verify the same nodes. Sampled
+    // verification is therefore not a source of noise, and without the flag we
+    // measure the incremental configuration users actually run. In
+    // non-incremental compiles the flag only triggered a costly query key
+    // verification sweep.
 
     if let Some(pos) = args.iter().position(|arg| arg == "--wrap-rustc-with") {
         // Strip out the flag and its argument, and run rustc under the wrapper